Dome cameras are generally categorized into two types: fixed-type dome cameras and movable-type dome cameras. The fixed-type dome camera has a fixed direction of a camera lens in a certain direction once the direction of the camera lens is manually set in this direction at the time of the installation, and the camera picks up a camera image in this set (fixed) direction. Specifically, in the fixed-type dome camera, the direction of the camera lens is adjustable in a certain angular range, so that the camera lens can be directed toward a target direction when the camera is installed. This angular range is an angular range where the direction of the camera lens can be adjustable during installing the camera, and may also be referred to as an “acceptable setting range”. Once the direction of the camera lens is set in a certain direction, the camera lens is fixed to be directed in this direction (set direction). Such a fixed-type dome camera may be used in surveillance in a predetermined area using a single camera.
In the movable-type dome camera, a camera lens is electrically adjustable (using a pan motor or a tilt motor, etc.) even after the camera is installed. Hence, the movable-type dome camera does not employ a manual setting of the direction of the camera lens, as the fixed-type dome camera does. Specifically, the direction of the camera lens is adjustable within a visible range to be directed in a desired direction after the camera is installed. The “visible range” denotes an angular range where the direction of the camera lens can be changed after the camera is installed, and the visible range of the movable-type dome camera may be set such that: “the pan direction: 360° (0° to 360°), the tilt direction: 75° (0° to 75°)”. Meanwhile, in the fixed-type dome camera, the direction of the camera lens is unchangeable after the camera is installed; therefore, the visible range may be set such that: “the pan direction: ±0°, the tilt direction: ±0°”. Not to mention that the concept of the “visible range” is quite different from the concept of the aforementioned “acceptable setting range”.
The fixed-type dome camera is relatively inexpensive, but there is limitation that a single camera can monitor only one area. To the contrary, the movable-type dome camera is relatively expensive, but multiple areas can be monitored using a single camera. As mentioned above, the dome cameras are categorized into two types: the fixed type and the movable type, and the present invention relates to the fixed-type dome camera having a camera lens whose direction is fixed manually.
Conventional fixed-type dome cameras employ semispherical dome covers. Taking the lens effect of a dome cover into account, a camera image in higher quality can be obtained by bringing an optical axis of the camera lens to pass through the spherical center of the dome cover. However, a conventional dome camera causes vignetting if the direction of its camera lens exceeds a certain angle (inclination limit angle) when the camera is set in the horizontal direction (horizontal direction if the summit direction of the dome is directed upward) or in the depression angle direction (more downward direction than the horizontal direction if the summit direction of the dome is directed upward) with its camera lens tilted. For this reason, the acceptable setting range of the conventional fixed-type dome camera is limited within a range of 0° (summit direction of the dome) to 75° (inclination limit angle) in the tilt direction.
There is suggested a conventional fixed-type dome camera that can tilt its camera lens at the depression angle direction (see Patent Literature 1, for example). In this conventional dome camera, the tilt direction of the camera lens in the horizontal direction or in the depression angle direction can be attained by deviating the position of an optical axis of the camera lens from a spherical center of a dome cover, which means that the tilt direction of the camera lens can be defined within a broader angular range.
Unfortunately, in such a conventional dome camera, the position of the optical axis of the camera lens is deviated from the spherical center of the dome cover if the tilt direction of the camera lens is set in the horizontal direction or in the depression angle direction; therefore, quality of a camera image becomes deteriorated due to the lens effect of the dome cover. Consequently, it is extremely difficult to cope with allowing the setting of the tilt direction of the camera lens in a broader angular range (even in the horizontal direction or in the depression angle direction) and realizing a camera image in high quality even if the tilt direction of the camera lens is set in the horizontal direction or in the depression angle direction.